The present invention relates generally to a servo system in a disc drive. More particularly, the present invention relates to developing and implementing a servo system for controlling microactuators in a disc drive to accomplish simultaneous data transfer from multiple heads on multiple discs relative to the discs, or to accomplish sequential data transfers or short track seeks without incurring servo overhead.
A typical disc drive includes one or more magnetic discs mounted for rotation on a hub or spindle. A typical disc drive also includes one or more transducers supported by a hydrodynamic air bearing which flies above each magnetic disc. The transducers and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs.
An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored on the magnetic discs by providing a write signal to the data head to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is then decoded by the drive controller to recover the data represented by flux reversals stored on a magnetic disc, and consequently represented in the read signal provided by the data head.
Accurate positioning of the data head over a track on the disc is of great importance in writing data to the disc and reading data from the disc.
In prior systems, servo operations were accomplished based on a dedicated servo head. In a dedicated servo type of system, servo information is all written to one dedicated surface of a disc in the disc drive. All of the heads in the disc drive are mechanically coupled to the servo head which is used to access the servo information. Thus, all of the heads in the dedicated servo disc drive are positioned based on the servo information read from the servo surface. This type of system allows the disc drive to conveniently execute parallel read and write operations. In other words, with appropriate circuitry in the drive controller, read and write operations can be executed in parallel using a plurality of the data heads mounted on the actuator, the data heads being simultaneously positioned based on the servo information read from the dedicated servo surface.
However, track densities on magnetic discs have been increasing for many years. Increased track densities on the magnetic disc require more accurate and higher resolution positioning. The mechanical offset between heads in a dedicated servo system can exceed one track width. Thus, the industry has seen a shift to embedded servo information in certain applications.
In an embedded servo system, servo information is embedded on each track on each surface of every disc. Thus, each data head returns a position signal independently of the other data heads. Therefore, the servo actuator is used to position each individual data head while that particular data head is accessing information on the disc surface. The positioning is accomplished using the embedded servo data for the track over which the data head is then flying.
While this results in increased positioning accuracy and higher resolution in the positioning process (because the data head is positioned independently of any other data heads), certain disadvantages are encountered because of increased track density and mechanics. One of the disadvantages is that in typical embedded servo systems, the ability to execute parallel read and write operations using a plurality of heads is lost. That is because the servo system is positioning based on information received by one individual data head, and the mechanical tolerances are inadequate to accurately position the other data heads in high track density systems. Also, current actuators are incapable of independently positioning the data heads. Thus, embedded servo systems, to date, have been unable to execute parallel read and write operations, such as simultaneously reading or writing a full cylinder in a disc drive.
Conventional servo actuators in conventional servo systems also present other disadvantages. For example, during a sequential reading or writing operation, the overhead associated with a servo operation is a significant cause for reduced throughput.
The present sequence for each sequential data transfer is to position a head over a data track, allow the disc drive controller to transfer all of the data to that track (which takes one revolution of the disc). Then the disc drive controller commands the servo processor to switch to the next head and position the servo actuator based on servo information retrieved from that head. The servo processor then electronically moves the servo actuator to position the new head over the new data track. When the new head is in position, the servo processor indicates to the controller that it can now continue transferring data.
The servo overhead in sequential operations is the time required for the servo actuator to position the next data head which will be writing or reading information to the center of the data track over which it is to read or write the information, from the position of the center of the track over which the previous data head was reading or writing information. Generally this is approximately 0.5 to 5 milliseconds. During this time, the sequential transfer of data is interrupted.
This overhead also prevents the provision of a continuous uninterrupted data stream when reading from and writing to the disc. This can be a significant disadvantage when reading and writing audio and video information, or when writing to a CD ROM from data stored on a hard disc drive.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.
The present invention relates to a system which is directed to solving one or more of the above-mentioned problems.
In accordance with one embodiment of the invention, a method is provided for transferring information relative to a plurality of surfaces of at least one disc in a disc drive. The surfaces have embedded servo information thereon. A plurality of data heads are provided, and each data head corresponds to one of the plurality of surfaces. The data heads are simultaneously positioned relative to the corresponding surfaces using the embedded servo information from the surfaces. Information is simultaneously transferred relative to the corresponding surfaces using the plurality of data heads.
The present invention may also be implemented as a method for controlling the position of a plurality of data heads relative to the corresponding disc surfaces. The method includes controlling movement of each of the plurality of data heads based on movement of the other of the plurality of data heads.
Further, the present invention can be implemented as a method of accomplishing sequential data transfers substantially without any servo overhead.
Also, the present invention can be implemented as an apparatus for accomplishing servo positioning in a disc drive. The apparatus includes a multiple input, multiple output, servo controller coupled to a plurality of microactuators. The microactuators are coupled to the data heads to position each of the data heads relative to the corresponding disc surfaces independently of one another. In another embodiment, the present invention includes a system which positions each of the data heads, using the microactuators, relative to a coarse servo actuator which is used to position all of the data heads relative to the disc surfaces.
The present invention further includes a model-based servo controller for controlling the coarse actuator and the microactuator.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading the following detailed description and review of the associated drawings.